Abstract

This pair deals with a new site of dolomite formation in supratidal evaporite flats, at the southernmost edge of the Ojo de Liebre evaporitic complex (Baja California, Mexico). The sedimentary context is mostly siliciclastic, except for calcareous outcrops around the flats. Both chemical and isotopic data on the interstitial solutions demonstrate a mixing between Mg (super 2+) -rich marine-derived brines and calcium bicarbonate-bearing continental waters, which are subjected to concentration by capillary evaporation. The lack of any in situ solid CaCO 3 in the sedimentary matrix argues for direct crystallization of dolomite from aqueous solutions. This is documented by the oxygen isotope compositions of the dolomite which are consistent with deposition under equilibrium conditions of ambient temperature and delta 1 8 O of water of the interstitial solutions. Furthermore, the low delta 13 C values of the dolomites and the absence of living plants or organisms in the basin indicate that a fraction of the total dissolved inorganic carbon reservoir is biogenic and is mostly provided by continental groundwaters. Therefore, crystallization of dolomite occurs within the sands infiltrated by solutions whose state of saturation relative to dolomite is mainly governed by the chemistry of continental groundwaters mixing with the inflowing marine brines.